Publications and other reference materials referred to herein are numerically referenced in the following text and respectively grouped in the appended Bibliography which immediately precedes the claims.
Phase shift interferometry (PSI) [1] is a well-established optical technology that allows high resolution non-contact three dimensional (3D) topographical measurements of objects. In PSI the 3D topography is obtained by using the interference signal phase values from each point of the surface of the object. As the phase is packed within the interference signal, there is a need to extract the phase. The standard approach to extract the phase from the interference signal is by changing the reference arm length of the interferometer of the PSI system in four (minimum three) equal steps with step size equivalent to π/2 phase shift [5]; for each step the interference signal is recorded. Using these four phase shifted signals the phase is extracted for each point on the surface of the sample.
There are several problems with the standard phase shift approach. First, as PSI is a very sensitive technique with sensitivity in the nanometer (nm) range, the fact that the phase shifted signals are grabbed successively and not simultaneously might introduce an error if the interferometer is not super stable during the successive phase shifted measurements. Second, dynamically changing scenes cannot be imaged without producing serious errors since the phase shifting requires time. Third, industrial rapid process control procedures are seriously slowed down because for each frame of the sample field four (minimum three) snapshots must be made in order to extract the phase. On top of all these, the extracted phase is wrapped 2π modulo so that surfaces with overall height variation larger than one fringe of the interference signal, e.g., a variation of 2π in the phase of the interference signal, cannot be easily reconstructed.
Generally speaking, a change of 2π in the phase of the interference signal is equivalent to a change of λ/2 in the surface topography. Obviously, this limits the technique to samples with height variations of only several hundreds of nanometers while the majority of applications require the ability to image samples with height variations of several micrometers. In the past, many researchers concentrated on phase unwrapping algorithms [3-9] which provide partial solution for samples with overall height variations larger than one fringe; however, these algorithms are often likely to collapse if the sample is with large steps, discontinuities or too large speckle noise. An important solution for these problems, which included two wavelengths, was suggested back in the seventies in the context of holography [10]; later this approach was implemented in PSI [11-13] and also using several wavelengths [14].
In the past, several research groups published simultaneous PSI using four CCD cameras [19-21] each grabbing an image with π/2 phase shift. However, to the best of the inventors' knowledge none of the preceding works demonstrated simultaneous PSI using only three CCDs. Moreover, none of the preceding works demonstrated multi wavelengths simultaneous PSI. In addition none of the preceding works demonstrated a three wavelengths calibration procedure to correct for the non-idealities of the optical phase components used.
It is a purpose of the present invention to provide a PSI method and system in which the phase shifted images are grabbed simultaneously and therefore do not require a specially-quiet environment nor ideal optical phase components.
It is another purpose of the present invention to provide a PSI method and system in which the phase shifted images are grabbed simultaneously at several wavelengths thereby allowing imaging samples with topographical variations larger than λ/2.
It is another purpose of the present invention to provide a PSI method and system in which the optical setups, as well as the algorithms, are completely different from those disclosed in previously proposed methods and systems.
It is another purpose of the present invention to provide a PSI method and system that overcomes the problems that have previously arisen using standard PSI system and method.
Further purposes and advantages of this invention will appear as the description proceeds.